1. Field of the Invention
The present invention relates to a planographic printing plate precursor that can be used as an offset printing master, and more particularly, it relates to a so-called positive-type planographic printing plate precursor for direct plate-making capable of forming a printing plate directly from digital data of a computer or the like.
2. Description of the Related Art
Advances in laser technology have been remarkable in recent years, and in particular, solid lasers and semiconductor lasers that have near infrared to infrared emitting regions, and that have high power and are compact are readily available. These laser devices are significantly useful as exposure light sources for direct plate-making from digital data of a computer or the like in the field of planographic printing.
A positive-type planographic printing plate precursor used with an infrared laser contains, as essential components, an alkali-soluble binder resin and an IR dye or the like which generates heat through light absorption. The IR dye or the like functions as a development inhibitor, which substantially decrease solubility of the binder resin in a developer through interaction with the binder resin, in an unexposed portion (i.e., an image area), and on the other hand, the interaction between the IR dye or the like and the binder resin is diminished in an exposed portion (i.e., a non-image area) to make the IR dye or the like dissolve in the alkali developer, whereby a planographic printing plate is formed.
Since the image forming ability of the positive-type planographic printing plate precursor used with the infrared laser depends on heat generation caused by irradiation with the infrared laser on a surface of a recording layer, the amount of heat used for forming images, i.e., solubilization of the recording layer, is lowered due to heat diffusion to the support in the vicinity thereof, to thereby lower the sensitivity. Therefore, a problem arises in that an effect for losing development inhibiting function of the recording layer is not sufficiently obtained in the non-image area, and consequently, the difference between the image area and the non-image area decreases to cause insufficient reproducibility in highlight portion.
In order to solve the problem associated with reproducibility in highlight portion, it has been proposed to use a recording layer comprising a material that allows the non-image area to be readily developed, i.e., that exhibits good solubility in an aqueous alkali solution. However, such a recording layer becomes low in chemical ability even in the image area to thereby pose a problem of deteriorated chemical resistance, such as becoming susceptible to damage by the action of the developer as well as an ink cleaner and a plate cleaner used upon printing. Accordingly, there has been a strong demand for a resin material that is excellent in chemical resistance and durability of a film prepared therefrom in the non-exposed area, and excellent in developing ability after having undergone light exposure and thereby losing solubility inhibiting function.
It is difficult to solve the aforementioned problems when a mono-layer type recording layer is adopted by selecting the alkali-soluble resin and other components. Thus, a planographic printing plate precursor is disclosed that has a recording layer having a lower layer which is excellent in alkali-solubility due to inclusion of a polyvinylphenol resin and a upper layer which contains a water-insoluble and alkali-soluble resin and an infrared absorber, and which exhibits enhanced solubility in an aqueous alkali solution through light exposure (see, for example, Japanese Patent Application Laid-Open (JP-A) No. 10-250255). However, this planographic printing plate precursor is still insufficient in chemical resistance, although sensitivity thereof is improved. Furthermore, there remains a problem in that adhesiveness between the support and the recording layer is insufficient, and hence printing durability is poor.
A variety of improved techniques have been proposed in order to attain similar objects, including that of making an image forming material by laminating a lower layer which contains a copolymer having a particular monomer and a photosensitive upper layer on a support (see, for example, JP-A No. 11-218914), and that of a process for producing a printing plate using a planographic printing plate precursor formed by laminating a lower layer which contains an alkali-soluble resin and an infrared sensitive and alkali developable upper layer on a hydrophilic support (see, for example, JP-A No. 11-194483). However, the former has a problem in that the resin used in the lower layer, although having good sensitivity and excellent chemical resistance, is insufficient in film strength to leave room for improvement in printing durability, while the latter has a problem in that, due to low chemical resistance of the alkali-soluble resin used, the recording layer is eluted with a solvent component included in a plate cleaner, and the solvent component penetrates into the interface between the lower layer and the support to thereby impair adhesiveness between the recording layer and the substrate, facilitating peelability of the recording layer. Thus, it has been difficult to satisfy both printing durability, which depends on the film strength of the lower layer, and chemical resistance.